The field of switching regulators is quite broad and includes every electronic circuit that performs a conversion of electric energy using a switching technique (hereinafter "converters").
In principle, the operation of switching regulators is based on the storage of electric energy by a reactive element. Periodically, with a given frequency, a fraction of the energy stored in the reactive element is transferred to a further reactive element, which in turn furnishes the electric energy stored therein to a load.
Switching regulators belong to two large families: AC-DC switching regulators, and DC-DC switching regulators. DC-DC switching regulators are typically used in applications comprised of several electronic sub-systems, requiring different supply voltages, with only a single DC voltage supply externally available. Thus, the different supply voltages are to be obtained starting from such single available DC voltage supply.
Generally, switching regulators include a control loop: an output electric quantity (output voltage or current) is monitored and compared to a reference quantity, corresponding to a target value for the output quantity. Depending on the "error", i.e., the difference between the monitored output quantity and the reference quantity, the frequency and/or duty cycle of opening/closing of a switching element are varied.
FIG. 1 depicts a schematic block diagram of a known DC-DC switching regulator, particularly a flyback switching regulator for generating a regulated DC negative voltage starting from a DC positive voltage. The regulator comprises a switching element 1, for example a P-channel MOS transistor, connected between a DC positive voltage Vin and a first terminal 2 of a first reactive element, namely an inductor L; the second terminal of the inductor L is connected to a reference voltage, namely the ground. The first terminal 2 of the inductor L is connected to a cathode of a diode D whose anode is connected to a first terminal 3 of a second reactive element, namely a first plate of a capacitor C; the second plate of capacitor C is connected to ground. An output terminal Vout of the regulator is connected to the first plate 3 of capacitor C. A feedback network 4 senses the voltage at the output terminal Vout and provides a corresponding signal 5 to an error amplifier 6. The error amplifier 6 comprises an operational amplifier 7, having an inverting input supplied with the signal 5 and a non-inverting input supplied with a reference voltage Vbg, for example generated by a band-gap reference voltage generator, not shown in the drawing. A compensation network 8 is connected in negative feedback to the operational amplifier 7. An output signal 9 of the error signal amplifier 6 is supplied to a positive input of a comparator 10; a negative input of the comparator 10 is supplied with a triangular signal 11 generated by a triangular signal generator 12. An output signal 13 of comparator 10 drives the switching element 1.
The described switching regulator is capable of providing, at the output terminal Vout thereof, a regulated DC negative voltage starting from the DC positive voltage Vin. The output voltage Vout is continuously monitored by the feedback network; the error amplifier 6 compares the output voltage Vout to the reference voltage Vbg and provides an error signal 9 proportional to the error, i.e., the difference between the output voltage Vout and the reference voltage Vbg. The comparator 10 and the triangular signal generator 12 form an error-to-duty cycle converter that converts the error signal 9 into a variable duty cycle of a periodic square wave, driving the switching element 1.
In operation, the output voltage Vout, after an initial, start-up transient following the regulator power-up during which the capacitor C is progressively charged, reaches a steady state value.
A problem affects the shown switching regulator, and more generally any PWM switching regulator that works in a control loop converting the error signal into a variable duty cycle of the driving signal for the switching element 1. The problem is that at the beginning of the start-up transient the error amplifier 6 is unbalanced, due to the fact that the output capacitor C is completely discharged. Therefore, the switching element 1 is kept almost always on, with a consequent electrical over-stress. This may cause the burning up, or at least the damage of the switching element.
This problem has already been addressed in the art. A so-called "soft start-up" capability has been provided to control the duty cycle of the switching element at the start-up of the switching regulator, in order not to destroy the switching element due to an excessive current.
The known solutions for implementing such a soft start-up capability are based on analog techniques that provide for applying a ramp voltage to the input or the output of the error amplifier.
The main problem with such techniques is the poor control of the activation time of the switching element due to the limited bandwidth of both the error amplifier and the error-to-duty cycle converter. As a consequence, the current flowing through the switching element is poorly controlled.
Another problem with the known technique is that, in order to achieve the desired long soft start-up times, relatively large capacitors are to be used for the generation of the ramp voltage. Such capacitors, due to their substantial dimensions, cannot be integrated on the same semiconductor chip where the switching regulator is integrated and must be provided as external components. This is obviously disadvantageous, at least because it imposes the provision of a dedicated pad for the connection of an external capacitor.